(Kind of like if your inflatable pool floats could get away from you. And were venomous.)

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At the mercy of winds and currents, many Portuguese man-of-wars wash ashore, like this one on the Canary Islands.

Photograph by Martin Siepmann, Alamy

Man-o-war tentacles can reach 165 feet (50 meters) long and are covered with nematocysts—“coiled, barbed bodies” that look like a squished Slinky, Burgess says. (See pictures of colorful sea creatures.)

On contact with prey, such as small fish, “boing! Out they come,” he says—along with an injection of disabling poison. Other parts of the tentacle pull the prey upward toward the mouth, where an injected enzyme starts the digestive process.

Saved by the Bell

One fish has hacked the system. The Portuguese man-of-war fish hangs out under the predator's "float" (called a bell), where there are fewer stingers, nibbling on its host's tentacles and “nutrient-rich reproductive organs."

The man-of-war has predators, too. Ocean sunfish, with their leathery skin, and loggerhead sea turtles, whose “mouths are hard as nails,” can chow down on them without getting stung, Burgess says.

For us humans, though, a man-of-war sting can cause welts and severe pain—and allergic reaction can affect a person's breathing, White says. "Drowning is a concern."

Burgess, an asthmatic, needed hospital treatment after a sting and advises simple caution in the ocean.

“Live your life,” he says, but “remember when you enter the sea that it’s a wilderness experience.”